🚀 go-pugleaf

In article <AGLEW.89Nov11193300@chant.urbana.mcd.mot.com> 
	aglew@urbana.mcd.mot.com (Andy-Krazy-Glew) writes:
>I'm a wide datapath proponent myself, but some of my contacts have
>responded that so many signals simulataneously changing state at the
>same time => huge instantaneous power demand.
>    
>How much of a problem is this *really*?

This is well-known as one of Seymour Cray's concerns. In the Cray-1,
he was worried that standing waves could develop in the copper ground
plane of a circuit board. His answer was to make a machine with power
demand that was (I believe) completely independant of the data
flowing through it. 

In the Cray-3, there is some reason to believe that the circuits will
have power demand that is also independent of the clock, that is,
absolutely constant rather than cyclic.

CMOS isn't like that. It's asymmetric, and cares about transitions:
0=>1 takes more power than 1=>1. So, one can write worst-case
programs, which generate on-chip noise (mass transitions on the wide
datapath), or which generate board noise and heat (mass transitions
on the address and data pins). I'm not sure what cache activity
generally produces the most heat: it may depend on implementation.

Another fun program is the one which puts many identical settings
into a content-addressable memory such as the TLB. Sixty four
registers all putting their hands up at once is enough to fratz
a naively designed TLB.

CMOS is wonderful, but the ECL/GaAs/BiCMOS folks talk an awful good
fight about how it's a different world on the other side of 50 MHz.

-- 
Don		D.C.Lindsay 	Carnegie Mellon Computer Science